Visual-Based Deficits

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Neurological factors that influence reading disabilities must have their immediate effect on cognitive-perceptual abilities that are not specific to reading because reading is an acquired skill. There is no aspect of cognition or a specific region of the brain that could fail to develop and just cause a reading disability (Ellis, 1985). If a reading disability is instrinsically motivated, it must be caused by differences in perceptual, cognitive, or linguistic abilities that have evolved to serve more primary human functions. We believe that the primary deficit underlying many reading disabilities is linguistic in nature. Later in this chapter, we will review the extensive body of research supporting the language basis of reading disabilities. First, however, we will consider the evidence that deficits in visual, auditory, or attentional processes play a causal role in reading disabilities.

Because the visual system is an important sensory system for reading, it should not be surprising that visual-based explanations of reading disabilities have a long history in the field (Bronner, 1917; Fildes, 1922; Frostig, 1968). Many early reported cases of reading disabilities were seen by ophthalmologists, who explained these problems in terms of visual difficulties. The term "word blindness" was frequently used to refer to reading disabilities. Several early clinics for reading difficulties also bore the name "Word Blind" in their title. Since these early accounts, there have been numerous attempts to uncover the visual deficits that might cause reading disabilities. These attempts have considered reversal errors, problems in visual memory, erratic eye movements, light sensitivity, and visual timing deficits.

Reversal Errors

Over the years, much attention has been focused on the reversal errors made by children with RD. These errors, which involve, for example, the reading/writing of b for d or was for saw, have traditionally been linked closely with dyslexia. Even today, most people still think of dyslexia as a problem reading letters or words backwards. Despite this view, there is surprisingly little research that has systematically investigated reversal errors. The few studies that have examined reversal errors have found that these errors do not actually occur that often in children with RD (Fischer, Liberman, & Shankweiler, 1978; Liberman, Shankweiler, Orlando, Harris, & Berti, 1971). Furthermore, when considered in terms of percentage of overall errors, reversal errors may be no more prevalent in young poor readers than they are in young good readers (Holmes & Peper, 1977). In other words, all beginning readers occasionally make reversal errors, just as all children learning to talk make errors involving grammatical morphemes (e.g., past tense -ed, third person -s). Just as children with language delays continue to have difficulty with grammatical morphemes beyond the developmental period, children with RD often continue to make reversal errors in later grades.

When reversal errors do occur, they generally are not the result of perceptual problems. Children who write saw as was or girl as gril typically do not have trouble perceiving letter sequences. Vellutino and his colleagues (Vellutino, Pruzek, Steger, & Meshoulam, 1973; Vellutino, Steger, DeSetto, & Phillips, 1975) found that children with RD could accurately copy what they sometimes failed to read correctly. Rather than having problems perceiving letter sequences, poor readers more likely have difficulties remembering the order of letters in words. Because of the spatial orientation of words, a primary way a word can be misspelled! misread is to fail to remember the correct order of its letters.

Visual Memory

Apparent problems in the memory for the letters in words led some early investigators to propose that poor readers had generalized deficits in visual memory (Fildes, 1922). Vellutino (1979), however, maintained that most of the early work showing deficits in visual memory was confounded by the use of stimuli that could be verbally labeled. Consequently, children with RD might have performed poorly because of verbal memory deficits rather than visual deficits. In support of this possibility, Vellutino and his colleagues (Vellutino et al., 1975) showed that poor readers scored comparably to good readers on a visual memory task involving stimuli that could not be easily labeled (but see Willows, Kruk, & Corcos, 1993).